Optical structure comprising a concave-and-convex-shaped interface between a high refractive-index layer and a low refractive-index layer and display device having the same

ABSTRACT

An optical structure includes a high refractive-index layer and a low refractive-index layer laminated on the high refractive-index layer and having a refractive index lower than that of the high refractive-index layer, and is disposed on a display surface of a display device. An interface between the layers has a concave-and-convex shape, and each of a concavity and a convexity in the shape has a flat portion extending in a surface direction of the layers. A side surface of the concave-and-convex shape, which extends between the flat portions of the concavity and convexity, is a curved surface or a folded surface that is convex to the low refractive-index layer. A difference between a maximum angle and a minimum angle, which are defined between the side surface of the concave-and-convex shape and a normal direction of the layers, is not less than 3 degrees and not more than 60 degrees.

FIELD OF THE INVENTION

The present invention relates to an optical structure that exerts anoptical effect on light emerged from a display surface of a displaydevice. In addition, the present invention relates to a display devicecomprising the optical structure.

BACKGROUND ART

A liquid-crystal display device as one example of display devices isused in various fields. A liquid crystal panel of the liquid-crystaldisplay device is roughly classified into a TN (Twisted Nematic) type, aVA (Vertical Alignment) type and an IPS (In-Plane Switching) type.

When a voltage is off, a liquid crystal panel of a TN type transmitslight, with liquid crystal molecules being oriented in a directionparallel to a display surface. By gradually increasing a voltage, theliquid crystal molecules are allowed to gradually stand up toward beingalong a normal direction of the display surface, a light transmittancegradually decreases. When a voltage is off, a liquid crystal pane of aVA type blocks light, with liquid crystal molecules being oriented alonga normal direction of a display surface. By gradually increasing avoltage, the liquid crystal molecules are gradually inclined towardbeing along the display surface, a light transmittance graduallyincreases. A liquid crystal panel of an IPS type adjusts a lighttransmittance by rotating liquid crystal molecules oriented along adisplay surface in accordance with application of voltage.

In a liquid crystal panel, it is generally important to control anamount and/or range of light that travels toward a front side, in orderto suitably ensure a brightness, a contrast ratio and colorreproducibility in a front view. On the other hand, to control lightwhich travels in a direction inclined to a normal direction of a liquidpanel is relatively complicated. Thus, in order to ensure a wide viewingangle and/or to sufficiently control a brightness, dispersions of acontrast ratio and color reproducibility in the viewing angle, astructure of the liquid panel may be complicated to undesirably increasecost. In order to deal with such a problem, Patent Documents 1 to 6, forexample, respectively disclose an optical member provided on a displaysurface of a liquid crystal panel in order to widen a viewing angle dueto a diffusion effect or the like. Such a member can simply improve aviewing angle.

Patent Document 1: JPH7-43704A

Patent Document 2: JP3272833B

Patent Document 3: JP3621959B

Patent Document 4: JP2016-126350A

Patent Document 5: JP2012-145944A

Patent Document 6: JP2011-118393A

SUMMARY OF THE INVENTION

In a liquid crystal panel employing the VA type among the above types,when a voltage to liquid crystal molecules is off, a black color isdisplayed. By bringing the black color very close to an actual blackcolor, a contrast ratio in a front view can be significantly gained. Onthe other hand, when a display surface of the liquid crystal panel isobserved from a direction inclined to a normal direction of the displaysurface, there is relatively a large amount of light leaked from a pixelthat is in a black color in a front view, so that a contrast ratio maybe seriously lowered as compared with the contrast ratio in a frontview. As a result, each contrast ratio in a viewing angle may largelyvary. When an optical member for merely diffusing light is provided onsuch a liquid crystal panel, the contrast ratio in a front view may beundesirably lowered, which impairs an advantage of a VA type. Similarly,the optical member for merely diffusing light may also undesirably lowera brightness in a front view.

In addition, when a display surface of a VA-type liquid crystal panel isobserved from a direction inclined to a normal direction of the displaysurface, an emission spectrum shape with respect to the inclineddirection changes so that color reproducibility lowers. The presentinventors have found that this is because the emission spectrum shapechange of displaying blue with respect to an observation angle isintensive (as compared with displaying red and displaying green). To bespecific, since “an intensity of a wavelength component corresponding togreen” changes to increase as compared with “an intensity of awavelength component corresponding to blue”, a displayed color tends toturn yellow. It is convenient that this problem can be solved by theaforementioned optical member. However, it cannot be said that theaforementioned prior art can effectively suppress a color changedepending on an observation angle.

In addition, an optical member as disclosed in Patent Documents 1 to 6is sometimes provided, outside a layer that exerts an optical effect onincident light, with a surface member that forms an outermost surface ona light emergent side. Such a surface member can function as aprotective layer, but may lower a brightness of light emerged from theoutermost surface to seriously lower a brightness on a high angle side.Thus, such a surface member may undesirably disturb an improved viewingangle due to the layer that exerts an optical effect.

The present invention has been made in view of the above circumstances.The object of the present invention is to provide an optical structurecapable of simply suppressing color change dispersion in a viewing anglewhile maintaining good display quality of a display device in a frontview, and a display device comprising such an optical structure. Inaddition, the object of the present invention is to provide an opticalstructure capable of simply suppressing color change dispersion in aviewing angle while maintaining good display quality of a display devicein a front view, and further capable of suppressing that a brightness ina viewing angle is undesirably lowered by a surface member that forms anoutermost surface on a light emergent side, and a display devicecomprising such an optical structure.

An optical structure according to the present invention is an opticalstructure to be disposed on a display surface of a display device,comprising: a high refractive-index layer; and a low refractive-indexlayer laminated on the high refractive-index layer, and having arefractive index lower than that of the high refractive-index layer;wherein: an interface between the high refractive-index layer and thelow refractive-index layer has a concave-and-convex shape; each of aconcavity and a convexity in the concave-and-convex shape has a flatportion extending in a surface direction of the high refractive-indexlayer and the low refractive-index layer; a side surface of theconcave-and-convex shape, which extends between the flat portion of theconcavity and the flat portion of the convexity, is a curved surface ora folded surface that is convex to the low refractive-index layer; thelow refractive-index layer is configured to be disposed to face thedisplay surface of the display device; and a difference between amaximum angle and a minimum angle, which are defined between the sidesurface of the concave-and-convex shape and a normal direction of thehigh refractive-index layer and the low refractive-index layer, is notless than 3 degrees and not more than 60 degrees.

In addition, an optical structure according to the present invention isan optical structure to be disposed on a display surface of a displaydevice, comprising: a high refractive-index layer; and a lowrefractive-index layer laminated on the high refractive-index layer, andhaving a refractive index lower than that of the high refractive-indexlayer; wherein: an interface between the high refractive-index layer andthe low refractive-index layer has a concave-and-convex shape; each of aconcavity and a convexity in the concave-and-convex shape has a flatportion extending in a surface direction of the high refractive-indexlayer and the low refractive-index layer; a side surface of theconcave-and-convex shape, which extends between the flat portion of theconcavity and the flat portion of the convexity, is a curved surface ora folded surface that is convex to the low refractive-index layer; thelow refractive-index layer is configured to be disposed to face thedisplay surface of the display device; and a ratio of a total length ofthe flat portions with respect to a length of one cycle of the concavityand the convexity of the concave-and-convex shape is not less than 0.50and less than 1.00.

In addition, an optical structure according to the present invention isan optical structure to be disposed on a display surface of a displaydevice, comprising: a high refractive-index layer; and a lowrefractive-index layer laminated on the high refractive-index layer, andhaving a refractive index lower than that of the high refractive-indexlayer; wherein: an interface between the high refractive-index layer andthe low refractive-index layer has a concave-and-convex shape; each of aconcavity and a convexity in the concave-and-convex shape has a flatportion extending in a surface direction of the high refractive-indexlayer and the low refractive-index layer; a side surface of theconcave-and-convex shape, which extends between the flat portion of theconcavity and the flat portion of the convexity, is a curved surface ora folded surface that is convex to the low refractive-index layer; thelow refractive-index layer is configured to be disposed to face thedisplay surface of the display device; and an average inclination angleof the side surface of the concave-and-convex shape, which is definedbetween a straight line connecting both end points of the side surfaceof the concave-and-convex shape and a normal direction of the highrefractive-index layer and the low refractive-index layer, is not lessthan 9 degrees and not more than 18 degrees.

In addition, an optical structure according to the present invention isan optical structure to be disposed on a display surface of a displaydevice, comprising: a high refractive-index layer; a lowrefractive-index layer laminated on the high refractive-index layer, andhaving a refractive index lower than that of the high refractive-indexlayer; and a surface member disposed on the high refractive-index layeron a side opposite to the low refractive-index layer; wherein: aninterface between the high refractive-index layer and the lowrefractive-index layer has a concave-and-convex shape; each of aconcavity and a convexity in the concave-and-convex shape has a flatportion extending in a surface direction of the high refractive-indexlayer and the low refractive-index layer; two of side surfaces of theconcave-and-convex shape, which are adjacent to each other and extendbetween the flat portion of the concavity and the flat portion of theconvexity, form a tapered shape tapering toward a direction in which theconcavity is recessed or a direction in which the convexity projects;the low refractive-index layer is configured to be disposed to face thedisplay surface of the display device; the surface member forms anoutermost surface on a side opposite to the display surface of thedisplay device; and a refractive index of the surface member is not morethan 1.40.

In the optical structure according to the present invention, the sidesurface of the concave-and-convex shape may be a curved surface or afolded surface that is convex to the low refractive-index layer.

In addition, in the optical structure according to the presentinvention, a difference between a maximum angle and a minimum angle,which are defined between the side surface of the concave-and-convexshape and a normal direction of the high refractive-index layer and thelow refractive-index layer, is preferably not less than 3 degrees andnot more than 60 degrees.

In addition, in the optical structure according to the presentinvention, a ratio of a total length of the flat portions with respectto a length of one cycle of the concavity and the convexity of theconcave-and-convex shape is preferably not less than 0.50 and less than1.00.

In addition, in the optical structure according to the presentinvention, an average inclination angle of the side surface of theconcave-and-convex shape, which is defined by a straight line connectingboth end points of the side surface of the concave-and-convex shape, anda normal direction of the high refractive-index layer and the lowrefractive-index layer, is preferably not less than 9 degrees and notmore than 18 degrees.

In addition, a display device according to the present invention is adisplay device in which any of the above-mentioned optical structure isdisposed on a display surface.

The display device according to the present invention may comprise: aliquid crystal panel having the display surface and a back surfaceopposed to the display surface; and a surface light source devicedisposed to face a back surface of the liquid crystal panel.

The liquid crystal panel may be a VA type liquid crystal panel which isconfigured such that, when a voltage to liquid crystal molecules is offor at a minimum value, the liquid crystal molecules are oriented along anormal direction of the display surface so that light from the surfacelight source device is blocked, and such that, when a voltage to theliquid crystal molecules is gradually increased, the liquid crystalmolecules are inclined little by little to being along the displaysurface so that a transmittance of the light from the surface lightsource device is gradually increased.

The present invention can simply suppress color change dispersion in aviewing angle, while maintaining good display quality of the displaydevice in a front view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a display device comprising anoptical structure according to one embodiment of the present invention.

FIG. 2 is a schematic sectional view of the display device forexplaining a behavior of light in the display device according to thisembodiment.

FIG. 3 is an enlarged sectional view of the optical structure accordingto this embodiment.

FIG. 4 is an enlarged view of a concave-and-convex shape formed on aninterface between a high refractive-index layer and a lowrefractive-index layer of the optical structure according to thisembodiment.

FIG. 5 is a view showing a plurality of side surfaces ofconcave-and-convex shapes of the optical structure, the side surfaceshaving curvatures different from one another.

FIG. 6 is a view showing behaviors of light reflected by the pluralityof side surfaces of the concave-and-convex shapes, the side surfaceshaving curvatures different from one another.

FIG. 7A is a view showing a graph showing color change in a viewingangle of light emitted from the optical structure through the displaydevice, in accordance with a curvature (a difference between a maximumangle and a minimum angle) of the side surface of the concave-and-convexshape of the optical structure.

FIG. 7B is a view showing a graph showing a degree of color change oflight emitted from the optical structure through the display device, inaccordance with a curvature (a difference between a maximum angle and aminimum angle) of the side surface of the concave-and-convex shape ofthe optical structure.

FIG. 8A is a view showing a graph showing a radiance in a viewing angleof light emitted from the optical structure through the display device,in accordance with a curvature (a difference between a maximum angle anda minimum angle) of the side surface of the concave-and-convex shape ofthe optical structure.

FIG. 8B is a view showing a graph showing a degree of lowering ofradiance of light emitted from the optical structure through the displaydevice, in accordance with a curvature (a difference between a maximumangle and a minimum angle) of the side surface of the concave-and-convexshape of the optical structure.

FIG. 9A is a view showing a graph showing a contrast in a viewing angleof light emitted from the optical structure through the display device,in accordance with a curvature (a difference between a maximum angle anda minimum angle) of the side surface of the concave-and-convex shape ofthe optical structure.

FIG. 9B is a view showing a graph showing a degree of lowering ofcontrast of light emitted from the optical structure through the displaydevice, in accordance with a curvature (a difference between a maximumangle and a minimum angle) of the side surface of the concave-and-convexshape of the optical structure.

FIG. 10 is a view showing a plurality of side surfaces ofconcave-and-convex shapes of the optical structure, the side surfaceshaving average inclination angles different from one another.

FIG. 11A is a view showing a graph showing color change in a viewingangle of light emitted from the optical structure through the displaydevice, in accordance with an average inclination angle of the sidesurface of the concave-and-convex shape of the optical structure.

FIG. 11B is a view showing a graph showing a degree of color change oflight emitted from the optical structure through the display device, inaccordance with an average inclination angle of the side surface of theconcave-and-convex shape of the optical structure.

FIG. 12A is a view showing a graph showing a radiance in a viewing angleof light emitted from the optical structure through the display device,in accordance with an average inclination angle of the side surface ofthe concave-and-convex shape of the optical structure.

FIG. 12B is a view showing a graph showing a degree of lowering ofradiance of light emitted from the optical structure through the displaydevice, in accordance with an average inclination angle of the sidesurface of the concave-and-convex shape of the optical structure.

FIG. 13A is a view showing a graph showing a contrast in a viewing angleof light emitted from the optical structure through the display device,in accordance with an average inclination angle of the side surface ofthe concave-and-convex shape of the optical structure.

FIG. 13B is a view showing a graph showing a degree of lowering ofcontrast of light emitted from the optical structure through the displaydevice, in accordance with an average inclination angle of the sidesurface of the concave-and-convex shape of the optical structure.

FIG. 14A is a view showing a graph showing color change in a viewingangle of light emitted from the optical structure through the displaydevice, in accordance with a ratio of flat portions of the theconcave-and-convex shape of the optical structure.

FIG. 14B is a view showing a graph showing a degree of color change oflight emitted from the optical structure through the display device, inaccordance with a ratio of flat portions of the the concave-and-convexshape of the optical structure.

FIG. 15A is a view showing a graph showing a radiance in a viewing angleof light emitted from the optical structure through the display device,in accordance with a ratio of flat portions of the theconcave-and-convex shape of the optical structure.

FIG. 15B is a view showing a graph showing a degree of lowering ofradiance of light emitted from the optical structure through the displaydevice, in accordance with a ratio of flat portions of the theconcave-and-convex shape of the optical structure.

FIG. 16A is a view showing a graph showing a contrast in a viewing angleof light emitted from the optical structure through the display device,in accordance with a ratio of flat portions of the theconcave-and-convex shape of the optical structure.

FIG. 16B is a view showing a graph showing a degree of lowering ofcontrast of light emitted from the optical structure through the displaydevice, in accordance with a ratio of flat portions of the theconcave-and-convex shape of the optical structure.

FIG. 17 is a schematic sectional view of a modification example of thedisplay device according to the embodiment.

FIG. 18 is a schematic sectional view of a modification example of thedisplay device according to the embodiment.

FIG. 19 is a schematic sectional view of a modification example of thedisplay device according to the embodiment.

FIG. 20 is a schematic sectional view of a modification example of theoptical structure according to the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention will be described herebelow withreference to the drawings.

In this specification, the terms “sheet”, “film”, “plate” and “layer”are not differentiated from one another, based only on the difference ofterms. Thus, for example, a “sheet” is a concept including a member thatcan be referred to as film, plate or layer. In addition, in thisspecification the term “sheet plane (plate plane, film plane)” means aplane corresponding to a plane direction (surface direction) of asheet-like member when it is seen as a target as a whole in general.Further, in this specification, a normal direction of a sheet-likemember is a direction normal to a sheet plane of a sheet-like member asa target.

A basic structure of a display device 10 comprising an optical structure100 according to one embodiment of the present invention is firstlydescribed with reference to FIGS. 1 to 4. FIG. 1 is a schematicsectional view of the display device 10 comprising the optical structure100. FIG. 2 is a sematic sectional view of the display device 10 forexplaining a behavior of light in the display device 10. FIG. 3 is anenlarged sectional view of the optical structure 100. FIG. 4 is anenlarged view of a concave-and-convex shape formed on an interfacebetween a high refractive-index layer and a low refractive-index layerof the optical structure 100. In the above respective sectional views,hatching is sometimes omitted for the convenience of explanation. Inaddition, FIGS. 1 to 4 show sectional views of a plane including abelow-described first direction d₁, and a common normal direction of aliquid crystal panel 15 of the display device 10 and a sheet-like basemember 101 of the optical structure 100. In this embodiment, the firstdirection d₁ is a direction parallel to a direction in which a lightsource 24 of a surface light source device 20 of an edge-light type inthe display device 10 emits light to a light guide plate 30, asdescribed below.

(Display Device)

An overall structure of the display device 10 is firstly described. Asshown in FIG. 1, the display device 10 according to this embodimentcomprises a liquid crystal panel 15, a surface light source device 20disposed to face a back surface 15B of the liquid crystal panel 15 so asto illuminate the liquid crystal panel 15 in a surface shape from theside of the back surface 15B, and a sheet-like optical structure 100disposed on a display surface 15A of the liquid crystal panel 15. Theliquid crystal panel 15 has the display surface 15A that displays astatic image or a dynamic image, and the back surface 15B opposed to thedisplay surface 15A. In the display device 10, the liquid crystal panel15 functions as a shutter that controls light transmission or block foreach area that forms a pixel (sub-pixel). An image is displayed on thedisplay surface 15A by driving the liquid crystal panel 15.

The illustrated liquid crystal 15 has an upper polarizing plate 13disposed on the light emergent side, a lower polarizing plate 14disposed on the light incident side, and a liquid crystal layer 12disposed between the upper polarizing plate 13 and the lower polarizingplate 14. The polarizing plates 14 and 13 each have a function fordividing incident light into two polarization components (e.g., P waveand S wave) orthogonal to each other, and allowing a linear polarizationcomponent (e.g., P wave), which oscillates in one direction (directionparallel to transmission axis), to transmit therethrough, whileabsorbing a linear polarization component (e.g., S wave), whichoscillates in the other direction (direction parallel to absorptionaxis) orthogonal to the one direction.

In the liquid crystal layer 12, a voltage can be applied to each areathat forms one pixel. Depending on whether a voltage is applied or not,an orientation direction of liquid crystal molecules in the liquidcrystal layer 12 changes. For example, when a polarization component ina predetermined direction, which has transmitted through the lowerpolarization plate 14 disposed on the light incident side, transmitsthrough the liquid crystal layer 12 to which no voltage is applied, thepolarization component rotates a polarization direction thereof at 90°.On the other hand, when such a polarization component transmits throughthe liquid crystal layer 12 to which a voltage is applied, thepolarization component maintains its polarization direction. In thiscase, by applying or not applying a voltage to the liquid crystal layer12, the polarization component, which has transmitted through the lowerpolarization plate 14 and oscillates in a predetermined direction, canbe controlled to further transmit through the upper polarization plate13 disposed on the light emergent side of the lower polarization plate14, or to be absorbed in the upper polarization plate 13 so as to beblocked. In this manner, in the liquid crystal panel 15, transmission orblock of light from the surface light source device 20 can be controlledfor each area that forms a pixel.

In this embodiment, the liquid crystal panel 15 is a VA (VerticalAlignment) type liquid crystal panel. Thus, the liquid crystal panel 15is configured such that, when a voltage to the liquid crystal moleculesin the liquid crystal layer 12 is off or at a minimum value, the liquidcrystal molecules are oriented along a normal direction of the displaysurface 15A so that light from the surface light source device 20 isblocked, and such that, when a voltage to the liquid crystal moleculesis gradually increased, the liquid crystal molecules are inclined littleby little to being along the display surface 15A so that a transmittanceof the light from the surface light source device 20 is graduallyincreased. However, the liquid crystal panel 15 is not limited to the VAtype, and may be of a TN (twisted Nematic) type liquid crystal panel oran IPS (In-Plane Switching) type liquid crystal panel. Details of theliquid crystal panel 15 are described in various known documents (e.g.,“Dictionary of Flat Panel Display (supervised by UCHIDA Tatsuo, UCHIIKEHeiji)” published in 2001 by Kogyo Chosakai Publishing Co., Ltd.), andfurther detailed description thereof is omitted.

Next, the surface light source device 20 is described. The surface lightsource device 20 has a light-emitting surface 21 that emits light in aplanar shape. In this embodiment, the surface light source device 20 isused as a device that illuminates the liquid crystal panel 15 from theside of the back surface 15B. As shown in FIG. 1, the surface lightsource device 20 is, e.g., an edge light type surface light sourcedevice, and has a light guide plate 30, a light source 24 disposedlaterally on one side (left side in FIG. 1) of the light guide plate 30,an optical sheet (prism sheet) 60 disposed to face the light guide plate30, and a reflection sheet 28 disposed to face the light guide plate 30.In the illustrated example, the optical sheet 60 directly faces theliquid crystal panel 15. A light emergent surface 61 of the opticalsheet 60 defines the light-emitting surface 21 of the surface lightsource device 20.

In the illustrated example, similarly to the display surface 15A of theliquid crystal panel 15 and the light-emitting surface 21 of the surfacelight source device 20, the light emergent surface 31 of the light guideplate 30 has a quadrangular shape in a plan view (shape seen fromabove). As a result, the light guide plate 30 is formed generally as aparallelepiped member in which thick-wise sides having a pair of mainsurfaces (light emergent surface 31 and back surface 32) are relativelysmaller than other sides. Side surfaces defined between the pair of twomain surfaces include four surfaces. Similarly, the optical sheet 60 andthe reflection sheet 28 are each formed generally as a parallelepipedmember in which thick-wise sides are relatively smaller than othersides.

As shown in FIGS. 1 and 2, the light guide plate 30 has theaforementioned light emergent surface 31 formed by one main surface onthe side of the liquid crystal panel 15, a back surface 32 formed by theother main surface opposed to the light emergent surface 31, and sidesurfaces extending between the light emergent surface 31 and the backsurface 32. One side surface of two surfaces of the side surfaces, whichis opposed to the first direction d₁, defines a light incident surface33. As shown in FIGS. 1 and 2, the light source 24 is disposed to facethe light incident surface 33. As shown in FIG. 2, light incident fromthe light incident surface 33 into the light guide plate 30 is guidedinside the light guide plate 30 generally along the first direction(light guide direction) d₁ toward an opposite surface 34, which isopposed to the light incident surface 33 along the first direction(light guide direction) d₁. The display device 10 according to thisembodiment is assumed to be positioned such that the first direction d₁is along the horizontal direction, i.e., the right and left direction.In this case, light from the light source 24 is guided in the light andleft direction. However, such a positioning is not particularly limited,and the display may be positioned otherwise. In this embodiment,although the surface light source device 20 is of an edge light type,the surface light source device 20 may be of another type such as adirect type or a back-illuminated type.

The light guide plate 30 is described in more detail. In thisembodiment, the back surface 32 of the light guide plate 30 is formed asa concave and convex surface. To be specific, as shown in FIG. 2, theback surface 32 has an inclined surface 37, a step surface 38 extendingin the normal direction of the light guide plate 30, and a connectionsurface 39 extending in a plate plane direction of the light guide plate30. Light guide in the light guide plate 30 is performed by a totalreflection effect by the pair of main surfaces 31, 32 of the light guideplate 30. On the other hand, the inclined surface 37 is inclined withrespect to the plate plane of the light guide plate 30 so as to comeclose to the light emergent surface 31 from the light incident surface33 toward the opposite surface 34. Thus, an incident angle of lighthaving been reflected by the inclined surface 37 into the pair of mainsurfaces 31, 32 is smaller. When the incident angle on the pair of mainsurfaces 31, 32 becomes less than a total reflection critical anglebecause of the reflection by the inclined surface 37, as shown by L1 ofFIG. 2, the light emerges from the light guide plate 30. Namely, theinclined surface 37 functions as an element for taking out light fromthe light guide plate 30. The light guide plate 30 is not limited to thetype in this embodiment, and may be of another type such as a dotpattern type.

The light source 24 may be formed of a fluorescent lamp such as a linearcold cathode tube, a dot-like LED (light-emitting diode), a filamentlamp and the like. The light source 24 in this embodiment is formed of alarge number of dot-like light-emitting elements, specifically, a largenumber of light-emitting diodes (LED), which are arranged side by sidealong the longitudinal direction of the light incident surface 33.

The reflection sheet 28 is a member disposed to face the back surface 32of the light guide plate 30. The reflection sheet 28 is a member forreflecting light leaked from the back surface 32 of the light guideplate 30 and for allowing the light to be again incident on the lightguide plate 30. The reflection sheet 28 is formed of a white diffusionreflection sheet, a sheet made of a material such as a metal having ahigh reflectance, a sheet including, as a surface layer, a membrane madeof a material having a high reflectance (e.g., metal membrane,dielectric multilayer membrane). Reflection by the reflection sheet 28may be either specular reflection (mirror reflection) or diffusereflection. When the reflection by the reflection sheet 28 is diffusereflection, the diffuse reflection may be either isotropic diffusereflection or anisotropic diffuse reflection.

The optical sheet 60 is a member having a function for changing atravelling direction of transmission light. As shown in FIG. 2, theoptical sheet 60 according to this example has a body part 65 formedlike a plate, and a plurality of unit prisms (unit shaped elements, unitoptical element, unit lenses) 70 formed on a light incident side surface67 of the body part 65. The body part 65 is formed as a flat plate-likemember having a pair of parallel main surfaces. In the illustratedexample, the unit prisms 70 are arranged side by side on the lightincident side surface 67 of the body part 65. Each unit prism 70 has acolumnar shape, and extends in a direction intersecting its arrangementdirection. Although one optical sheet 60 is provided on the light guideplate 30 in this embodiment, a plurality of optical sheets may beprovided on the light guide plate 30. In this case, orientations ofgrooves of prisms of the respective optical sheets may differ from oneanother.

Since the surface light source device 20 as described above comprisesthe optical sheet 60, the surface light source device 20 converts atraveling direction and a polarization state of light from the lightguide plate 30 into desired ones, and allows the light to be incident onthe liquid crystal panel 15. As described above, transmission or blockof the light incident on the liquid crystal panel 15 is controlled inthe liquid crystal layer 12 for each area that forms a pixel, dependingon whether a voltage is applied or not. Thus, an image is displayed onthe display surface 15A of the liquid crystal panel 15.

(Optical Structure) Next, the optical structure 100 is described indetail with reference to FIGS. 2 to 4. As shown in FIGS. 2 and 3, theoptical structure 100 according to this embodiment comprises: asheet-like or film-like base member 101 having an light emergent surface101A and a back surface 101B opposed to the light emergent surface 101A;a sheet-like or film-like high refractive-index layer 102 extendingalong the base member 101; a sheet-like or film-like lowrefractive-index layer 103 which is provided on a surface of the highrefractive-index layer 102, which surface is opposed to the side of thebase member 101, and extends along the base member 101, the lowrefractive-index layer 103 having a refractive index lower than that ofthe high refractive-index layer 102; and a sheet-like or film-likeantireflection layer 104 which is disposed on a side of the highrefractive-index layer 102, which side is opposed to the side of the lowrefractive-index layer 103. In this example, the antireflection film 104is provided on the light emergent surface 101A of the base member 101.The antireflection layer 104 in this embodiment is a membercorresponding to a surface member that forms an outermost surface of thelight emergent side.

The base member 101 is a transparent base member made of resin or glassand having a light transmission property. A material thereof may be, forexample, polyethylene terephthalate, polyolefin, polycarbonate,polyacrylate, polyamide, glass, triacetyl cellulose, and the like. Theoptical structure 100 is disposed such that the low refractive-indexlayer 103 faces the display surface 15A of the display device 10. In theillustrated example, the low refractive-index layer 103 is in directlycontact with the display device 10, i.e., the display surface 15A. Inaddition, the antireflection layer 104 forms an outermost surface on theside opposed to the side of the display surface 15A of the displaydevice 10. The antireflection layer 104 is provided for suppressingsurface reflection of outside light that is incident on the opticalstructure 100. Thus, it can be prevented that visibility of an imagedisplayed on the display device 10 is impaired by surface reflection ofoutside light.

A refractive index of the antireflection layer 104 is not more than1.40. More specifically, it is 1.35. A general refractive index of asurface member, such as an antireflection layer, is greater than about1.45 and is not more than about 1.50. A member having such a refractionindex is available relatively at a low price, which reduces cost.However, when a surface member, which has a refractive index greaterthan about 1.45 and not more than about 1.50, is used, a critical angle,at which light that is going to emerge from the display device starts tototally reflect at an interface between the surface member and air, isrelatively small. Thus, an amount of light that can be taken outdecreases. As a result, when an image on the display device is observed,a brightness of the image may lower, in particular, a brightness on ahigh angle side may significantly lower. In contrast thereto, in thisembodiment, a refractive index of the antireflection layer 104 is set tobe not more than 1.40, so as to raise a critical angle at which lightstarts to totally reflect at an interface between the antireflectionlayer 104 and air. Thus, an amount of light that can be taken outincreases as compared with a case of a general surface member.Accordingly, undesirable lowering of a brightness in a viewing angle issuppressed.

The present inventors have conducted intensive studies and have foundthat, in order both to prevent surface reflection of outside light andto take out sufficient light from the display device, a refractive indexof the antireflection layer 104 serving as a surface member ispreferably not less than 1.28 and not more than 1.40, and particularlypreferably not less than 1.30 and not more than 1.36 based onexperiments and simulations. In this embodiment, the antireflectionlayer 104 as a surface member has a function for suppressing surfacereflection of outside light, but the surface member may not have such afunction. In addition, in the optical structure 100, a color changesuppression effect in a viewing angle is obtained by an optical effectexerted by the high refractive-index layer 102 and the lowrefractive-index layer 103 on light, which is described below. However,such an effect can be obtained without the antireflection layer 104.

As shown in FIG. 3, in this embodiment, the high refractive-index layer102 has a plurality of lens parts 110 on a surface opposed to the basemember 101. The lens parts 110 are formed to be convex to the lowrefractive-index layer 103 along the normal direction of the highrefractive-index layer 102. Namely, the high refractive-index layer 102integrally has a film-like layer body 102A having a front surface facingthe base member 101 and a back surface opposed to the front surface toface the low refractive-index layer 103, and the plurality of lens parts110 arranged side by side on the back surface of the layer body 102A. Onthe other hand, the low refractive-index layer 103 is laminated on thehigh refractive-index layer 102 so as to cover the lens parts 110 and tofill spaces between the lens parts 110. Thus, in this embodiment, aninterface between the high refractive-index layer 102 and the lowrefractive-index layer 103 has a concave-and-convex shape 120.

In the concave-and-convex shape 120, one concavity 121 and one convexity122 form one cycle shape. The concave-and-convex shape 120 is formed byrepeatedly forming the one cycle shape. A portion which is recessed tothe side of the high refractive-index layer 102 with respect to areference line SL extending in a surface direction that passes amidpoint between a bottom of the concavity 121 and a top of theconvexity 122, corresponds to the concavity 121. A portion whichprojects to the side of the low refractive-index layer 103 with respectto the reference line SL corresponds to the convexity 122. Theconcavities 121 and the convexities 122 are respectively arranged in thefirst direction d₁, and linearly extend in a direction not parallel tothe first direction d₁, for example, in a direction orthogonal to thefirst direction d₁. The respective concavities 121 and the convexities122 in this example linearly extend in a direction orthogonal to thefirst direction d₁.

As shown in FIG. 3, each of the concavity 121 and the convexity 122 inthis embodiment has a flat portion 121A, 122A extending along a surfacedirection of the high refractive-index layer 102 and the lowrefractive-index layer 103. In detail, the bottom of the concavity 121defines the flat portion 121A, and the top of the convexity 122 definesthe flat portion 122A. In addition, a side surface 120S of theconcave-and-convex shape 120, which extends between the flat portion121A of the concavity 121 and the flat portion 122A of the convexity122, is a curved surface that is convex to the low refractive-indexlayer 103. The side surface 120S is formed so as not to extend, in thesurface direction, beyond a straight line extended from an end point ofthe flat portion 121A, to which the side surface 120S is connected,along the normal direction. Thus, die cutting of the highrefractive-index layer 102 having the lens parts 110 forming the sidesurfaces 120S is enabled. Although the side surface 120S is a curvedsurface in this embodiment, the side surface 120S may be a foldedsurface (polygonal shape) that is convex to the low refractive-indexlayer 103. In addition, the side surface 120S formed as a curved surfacemay be formed along an arc of a precise circle, or may be formed alongan arc of an ellipse. The concave-and-convex shape 120 as describedabove is provided for improving display quality of an image displayed onthe display surface 15A, by exerting an optical effect, such as totalreflection, refraction, transmission, etc., on light for displaying animage, which is emerged from the display surface 15A. In thisembodiment, since the side surface 120S is a curved surface convex tothe low refractive-index layer 103, two side surfaces 120S adjacent toeach other form a tapered shape tapering toward in a direction in whichthe concavity 121 is recessed or a direction in which the convexity 122projects. In other words, the side surfaces 120S, which are adjacent toeach other with the flat portion 121A of the concavity 121 beinginterposed therebetween, form a tapered shape tapering from the lowrefractive-index layer 103 toward the high refractive-index layer 102.In addition, the side surfaces 120S, which are adjacent to each otherwith the flat portion 122A of the convexity 122 being interposedtherebetween, form a tapered shape tapering from the highrefractive-index layer 102 toward the low refractive-index layer 103.With such a shape, an optical effect for improving display quality of animage displayed on the display surface 15A is generated. When the sidesurface 120S is a curved shape or a polygonal shape convex to the lowrefractive-index layer 103 as in this embodiment, color change in aviewing angle can be particularly effectively suppressed.

In addition, in this embodiment, the high refractive-index layer 102 andthe low refractive-index layer 103 are selected such that a differencebetween a refractive index of the high refractive-index layer 102 and arefractive index of the low refractive-index layer 103 is within a rangeof not less than 0.05 and not more than 0.25. In addition, the highrefractive-index layer 102 is disposed to direct front-side of thedisplay device 10, and the low refractive-index layer 103 is disposed toface the display surface 15A of the liquid crystal panel 15. In theillustrated example, the low refractive-index layer 103 is an adhesivelayer. As shown in FIG. 2, the optical structure 100 is joined to thedisplay surface 15A of the liquid crystal panel 15 by means of the lowrefractive-index layer 103. These high refractive-index layer 102 andthe low refractive-index layer 103 are also members having a lighttransmission property, and their material is not particularly limited.

FIG. 4 is an enlarged view showing the concave-and-convex shape 120.Herebelow, the concave-and-convex shape 120 is described in more detailwith reference to FIG. 4. A symbol θ1 shows a minimum angle that isdefined between the side surface 120S of the concave-and-convex shape120 and the normal direction of the high refractive-index layer 102 andthe low refractive-index layer 103. A symbol θ2 shows a maximum anglethat is defined between the side surface 120S of the concave-and-convexshape 120 and the normal direction of the high refractive-index layer102 and the low refractive-index layer 103. In more detail, the minimumangle θ1 is an angle that is defined between a tangent passing throughan end point of the side surface 120 s of the concave-and-convex shape120 on the side of the concavity 121 and the normal direction of theinterface between the high refractive-index layer 102 and the lowrefractive-index layer 103. The maximum angle θ2 is an angle that isdefined between a tangent passing through an end point of the sidesurface 120S on the side of the convexity 122 and the normal directionof the interface between the high refractive-index layer 102 and the lowrefractive-index layer 103. When the side surface 120S is a foldedsurface, the minimum angle θ1 is an angle that is defined between astraight line passing through an element surface including the end pointof the side surface 120S on the side of the concavity 121 and the normaldirection of the interface between the high refractive-index layer 102and the low refractive-index layer 103, and the maximum angle θ2 is anangle that is defined between a straight line passing through an elementsurface including the end point of the side surface 120S on the side ofthe convexity 122 and the normal direction of the interface between thehigh refractive-index layer 102 and the low refractive-index layer 103.In addition, a symbol θ0 shows an “average inclination angle” of theside surface 120S, which is defined between a straight line connectingboth end points of the side surface 120S of the concave and the convexshape 120 and the normal direction of the high refractive-index layer102 and the low refractive-index layer 103. A symbol P shows a pitchthat is an interval of one cycle composed of one concavity 121 and oneconvexity 122 of the concave-and-convex shape 120. A symbol H shows aheight of the concave-and-convex shape 120 from the concavity 121 up tothe convexity 122 along the normal direction. A symbol L shows adistance between both the end points of the side surface 120S in thesurface direction.

An “inclination angle range α” is defined by a difference between themaximum angle θ2 and the minimum angle θ1. The larger the inclinationangle range α is, the larger a curvature of the side surface 120S is.The present inventors have conducted intensive studies and have foundthat this inclination angle range α is preferably not less than 3degrees and not more than 60 degrees. In addition, the present inventorshave found that the aforementioned “average inclination angle θ0” ispreferably not less than 9 degrees and not more than 18 degrees.Further, the present inventors have hound that, when a ratio of a totallength of the flat portions 121A, 122A with respect to a length of onecycle of the concavity 121 and the convexity 122 of theconcave-and-convex shape 120 is represented as β, β=(a+b)/P isparticularly preferably not less than 0.60 and not more than 0.90, asshown in FIG. 4. The above “a” is a width (length) of the flat portion122A of the convexity 122, and the above “b” is a width (length) of theflat portion 121A of the concavity 121. Moreover, the present inventorshave found that a particularly preferred range of the inclination anglerange α varies depending on the ratio β. For example, as describedlater, when the ratio β is 0.80, it has been found that the inclinationangle range α is particularly preferably not less than 9 degrees and notmore than 16 degrees. However, when the ratio β varies, the particularlypreferred range of the inclination angle range α also varies.

When the concave-and-convex shape 120 is formed to simultaneouslysatisfy the aforementioned three kinds of dimensional conditions, colorchange in a viewing angle can be significantly effectively suppressed,while maintaining good display quality of the display device 10 in afront view. However, even when the above dimensional conditions is onlypartially satisfied, the concave-and-convex shape 120 can effectivelysuppress color change in a viewing angle.

Next, an example describing how to obtain that the aforementionedinclination angle range α, the average inclination angle θ0 and theratio β are in particularly preferred ranges is described.

(Relationship Between Inclination Angle Range (Curvature) of SideSurface of Concave-and-Convex Shape and Color Change)

Firstly, by way of example, the fact that, when the ratio β is 0.80, theparticularly preferred range of the inclination angle range α is notless that 9 degrees and not more than 16 degrees is described. FIG. 5shows a plurality of (three) side surfaces 120S of theconcave-and-convex shape 120, which have curvatures different from oneanother. The inclination angle range α of the side surface 120Sincreases from the left to the right in FIG. 5. Namely, the curvaturealso increases. When the curvature of the side surface 120S differs, abehavior of light which enters from the flat portion 122A of theconvexity 122 so as to be totally reflected by the side surface 120Svaries, as shown by a light trajectory LT in FIG. 6. When the sidesurface 120S is a curved surface, color change in a viewing angle can beparticularly effectively suppressed. Here, note that, in FIG. 6,although the left side surface 120S has the inclination angle range α of0 degrees so that its radius of curvature is infinite (Inf), the sidesurface 120S having the inclination angle range α of 0 degrees is notincluded in a “curved surface” concept, such a side surface 120S isshown for the sake of convenience of explanation. However, even thisside surface 120S having the inclination angle range α of 0 degrees cangenerate an optical effect for suppressing color change in a viewingangle. The center side surface 120S in FIG. 5 has the inclination anglerange α of 12 degrees, and its radius of curvature is 55 μm. The rightside surface 120 in FIG. 5 has the inclination angle range α of 22degrees, and its radius of curvature is 31 μm. In FIGS. 5 and 6, in thedifferent concave-and-convex shapes 120, the distance L between both endpoints of the side surface 120S in the surface direction is fixed as acertain value, and an inclination length as a distance connecting theboth end points of the side surface 120S by a straight line is alsofixed as a certain value. In addition, the width “a” of the flat portion122A of the convexity 122 is fixed as a certain value.

FIGS. 7A and 7B are graphs showing color change in a viewing angle ofoptical structures 100 corresponding to the three concave-and-convexshapes 120 (α=0 degrees, 12 degrees, 22 degrees) shown in FIG. 6. To bespecific, FIGS. 7A and 7B are graph showing color change of lightemerged from the optical structures 100 corresponding to theaforementioned respective concave-and-convex shapes 120, when the ratioβ, which is a ratio of a total length of the flat portions 121A, 122Awith respect to a length of one cycle of the concavity 121 and theconvexity 122 of the concave-and-convex shape 120, is 0.80. FIG. 7A is agraph in which an axis of abscissa shows an angle of light in a viewingangle, the light being emerged from the optical structure 100, and anaxis of ordinate shows color change Δu′v′. FIG. 7A shows color change ina viewing angle of light that was incident from the body (the side ofthe liquid crystal panel 15) of the display device 10 and was emergedfrom the optical structures 100 under the aforementioned conditions.When the angle shown by the axis of abscissa is 0 degrees (deg), itmeans that light is observed along the normal direction. For example, inthe case of 30 degrees, light was observed along a direction inclined at30 degrees with respect to the normal direction. In addition, the colorchange Δu′v′ means a color difference, which is calculated from colorsdefined by u′ and v′ in a uniform color space. A value of Δu′v′ at anangle θ in a certain viewing angle is expressed by the followingExpression (1).Δu′v′(θ)=√{square root over ((u′(θ)−u′(0))²−(v′(θ)−v′(0))²)}  (1)

The above u′ and v′ as color coordinates in a uniform color space in theExpression (1) are expressed by the following Expressions (2-1) and(2-2).

$\begin{matrix}{u^{\prime} = \frac{4x}{{{- 2}x} + {12y} + 3}} & \left( {2\text{-}1} \right) \\{v^{\prime} = \frac{9y}{{{- 2}x} + {12y} + 3}} & \left( {2\text{-}2} \right)\end{matrix}$

In the above respective expressions, x and y are color coordinatesdefined by CIE1931 color space (CIExyY color space).

FIG. 7B is a graph in which an axis of abscissa shows the inclinationangle range α, and an axis of ordinate shows a color change score. FIG.7B shows color change scores of light that was incident from a body (theside of the liquid crystal panel 15) of the display device 10 and wasemerged from the optical structures 100 under the aforementionedconditions. The color change score is a barometer which shows that, whena value thereof is smaller, color change is more considerably andeffectively suppressed in all the range of a viewing angle of 0 to 60degrees, as compared with a case in which the optical structure 100 isnot provided. The color change score is calculated by the followingExpression (3). In the Expression (3), θ shows an angle in the viewingangle, Film means a case where the optical structure 100 is provided onthe display device 10, and NonFilm means a case where the opticalstructure 100 is not provided on the display device 10. The color changescore is a barometer that is uniquely made by the present inventors inorder to evaluate a color change degree. As long as the color changeΔu′v′ can be specified, the color change score can be used forevaluating a member similar to the optical structure 100 according tothis embodiment.

$\begin{matrix}{{{Color}\mspace{11mu}{Change}\mspace{14mu}{Score}} = {\sum\limits_{\theta = 0}^{60}\frac{{{{\Delta\; u^{\prime}{v^{\prime}(\theta)}} - {\Delta\; u^{\prime}{v^{\prime}\left( {\theta + 5} \right)}}}}_{Film}}{{{{\Delta\; u^{\prime}{v^{\prime}(\theta)}} - {\Delta\; u^{\prime}{v^{\prime}\left( {\theta + 5} \right)}}}}_{NonFilm}}}} & (3)\end{matrix}$

In the color change evaluation shown in FIGS. 7A and 7B, a brightnessevaluation shown in FIGS. 8A and 8B and a contrast evaluation shown inFIGS. 9A and 9B, which are described later, a multi-domain type VA typeliquid crystal display device manufactured by Sony Corporation was usedas the body of the display device on which the optical structure 100 wasprovided. A blue image was displayed on the body of the display deviceby a pattern generator. Color change when the image was displayed on thebody of the display device without the optical structure 100, and colorchange when the image was displayed thereon with the optical device 100were evaluated by using a “spectroradiometer SR-2” manufactured byTOPCON Corporation. Below described evaluations shown in FIGS. 11A to13B and evaluations shown in FIGS. 13A to 16B were carried out under thesame conditions as described above.

In view of FIG. 7A created as described above, it can be understoodthat, when the display device 10 was provided with the optical structure100 in which the concave-and-convex shape 120 having the side surface Swith the inclination angle range α of 0 degrees, 12 degrees or 22degrees, color change in a viewing angle was suppressed, as comparedwith the display device 100 which was not provided with the opticalstructure 100. On the other hand, in the case where the inclinationangle range α is 0 degrees, within a range of 30 to 45 degrees of aviewing angle, it cannot be said that the color change graph smoothlytransitions in a viewing angle. From this tendency, the presentinventors have found that, when the inclination angle range α is toosmall, the diffusion effect is weak so that the color change suppressioneffect may be insufficient. In FIG. 6, in the case of the side surface120S having the inclination angle range α of 0 degrees, the lighttotally reflected by the side surface 120S emerges in a fixed angledirection. Thus, because of the phenomenon in which an angle at whichlight is diffused is small, it is considered that, when the inclinationangle range α is too small, the diffusion effect is weak so that thecolor change suppression effect tends to be insufficient.

In the case of the inclination angle range α of 22 degrees, it can beunderstood that the color change suppression effect is weak within arange of 30 to 45 degrees in a viewing angle, as compared with the othercases. From this tendency, the present inventors have found that, whenthe inclination angle range α is too large, the diffusion effect is weakso that the color change suppression effect may be insufficient. In FIG.6, in the case of the side surface 120S having the inclination anglerange α of 22 degrees, the light hits onto the side surface 120S at anangle not less than a critical angle, and refracts to escape. Because ofsuch a phenomenon, it is considered that, when the inclination anglerange α is too large, the diffusion effect is weak so that the colorchange suppression effect tends to be insufficient.

On the other hand, the color change score in FIG. 7B, which iscalculated by the Expression (3), is a barometer which shows that, whena value thereof is smaller, color change is more considerably and moresmoothly suppressed in a viewing angle. In view of FIG. 7B, it can beunderstood that, when the inclination angle range α is within a range ofnot less than 9 degrees and not more than 16 degrees, the color changescore tends to be remarkably suppressed. When the inclination anglerange α is within a range of not less than 7 degrees and not more than20 degrees, the color change score tends to be lower than the colorchange score outside this range. Thus, it can be said that theinclination angle range α of not less than 7 degrees and not more than20 degrees is preferred in terms of color change suppression. However,when the inclination angle range α is within a range of not less than 9degrees and not more than 16 degrees, the value of the color changescore is particularly low relatively, it can be said that such a rangeis particularly preferred. From this point and the respective findingsabout the above-described difference in the color change, it can beconcluded that, when the inclination angle range α of the side surface120S is within a range of not less than 9 degrees and not more than 16degrees, color change dispersion in a viewing angle is significantlysuppressed. FIG. 7B also shows a color change score in the opticalstructure 100 in which the inclination angle range α is different fromones illustrated in FIG. 7A.

From the above, the present inventors have found that, as an example,when the ratio β of the flat portions 121A, 122A with respect to onecycle of the concave-and-convex shape 120 is 0.80, a preferred range ofthe inclination angle range α is not less than 7 degrees and not morethan 20 degrees, a particularly preferred range thereof is not less than9 degrees and not more than 16 degrees. It was practically confirmedthat, with such a range, the color change dispersion in a viewing anglecould be effectively suppressed, as compared with a range outside thisrange. In the graph of FIG. 7B, when the inclination angle range αexceeds a position of 9 degrees, the color change score steeplydecreases, when the inclination angle range α falls below a position of16 degrees, the color change score steeply decreases, and at theposition of 16 degrees, the value of the color change score isrelatively sufficiently decreases as compared with a position of largerdegrees, so that criticality can be found at the respective positions.In the aforementioned preferred inclination angle range α, the points atwhich the color change score steeply decreases are set as a lower limitvalue and an upper limit value. Note that the inclination angle range αof not less than 9 degrees and not more than 16 degrees is preferred,and the inclination angle range of not less than 10 degrees and not morethan 15 degrees is more preferred.

In addition, the present inventors have found that, when the value ofthe ratio β is varied to the plus side from 0.80, the range in which thecolor change score steeply decreases, as shown in FIG. 7B, tends toshift in the plus direction of the inclination angle degree a, whilespreading in the axis of abscissa, and that, when the value of the ratioβ is varied to the minus side from 0.80, the range in which the colorchange score steeply decreases, as shown in FIG. 7B, tens to shift inthe minus direction. Thus, when the ratio β is 0.80 as described above,the particularly preferred inclination angle range α is not less than 9degrees and not more than 16 degrees. However, when the ratio β varies,such a preferred range also varies.

FIG. 8A is a graph showing a radiance at a wavelength of 450 nm in aviewing angle of light which was incident from the body (the side of theliquid crystal panel 15) of the display device 10 and emerged from theoptical structure 100. An axis of abscissa shows an angle in a viewingangle, and an axis of ordinate shows a radiance. FIG. 8B is a graphshowing a degree of lowering of a radiance of light which was incidentfrom the body (the side of the liquid crystal panel 15) of the displaydevice 10 and emerged from the optical structures 100 under theaforementioned conditions. An axis of abscissa shows a value of theinclination angle range α, and an axis of ordinate shows a radiancelowering rate (written as brightness lowering rate in FIG. 8B). Theradiance lowering rate is a barometer which shows that, when a valuethereof is smaller, a degree of lowering of a radiance is smaller, ascompared with a case in which the optical structure 100 is not provided.

In addition, FIG. 9A is a graph showing a contrast in a viewing angle oflight which was incident from the body (the side of the liquid crystalpanel 15) of the display device 10 and emerged from the opticalstructure 100. An axis of abscissa shows an angle in a viewing angle,and an axis of ordinate shows a contrast. FIG. 9B is a graph showing adegree of lowering of contrast of light which was incident from the body(the side of the liquid crystal panel 15) of the display device 10 andemerged from the optical structures 100 under the aforementionedconditions. An axis of abscissa shows a value of the inclination anglerange α, and an axis of ordinate shows a contrast lowering rate. Thecontrast lowering rate is a barometer which shows that, when a valuethereof smaller, a degree of lowering of contrast is smaller, ascompared with a case in which the optical structure 100 is not provided.

When the aforementioned inclination angle range α is within a range ofnot less than 9 degrees and not more than 16 degrees, it can beconcluded that lowering of a brightness and a contrast is not excessive,as compared with a case in which the optical structure 100 is notprovided. Thus, within this range, color change in a viewing angle canbe effectively suppressed, while maintaining good display quality of thedisplay device 10 in a front view. Also from this point, it can be saidthat the inclination angle range α is preferably not less than 9 degreesand not more than 16 degrees. Note that such a numerical range isnothing more than an example of a particularly preferred range, and thatthe present invention can achieve a useful effect even with a numericalrange different from the illustrated one.

(Relationship Between Average Inclination Angle of Side Surface ofConcave-and-Convex Shape and Color Change)

Next, the reason why the average inclination angle θ0 of the sidesurface 120S of the concave-and-convex shape 120 is preferably not lessthan 9 degrees and not more than 18 degrees is described. FIG. 10 showsa plurality of (three) side surfaces 120S of the concave-and-convexshape 120, which have different average inclination angle θ0. As theside surface 120S is inclined rightward in FIG. 10, it means that theaverage inclination angle θ0 increases. The respective side surfaces120S are set to have the same curvature with each other (i.e., a isfixed). FIGS. 11A and 11B show graphs showing color change in a viewingangle of optical structures 100 corresponding to the concave-and-convexshapes 120 in which the average inclination angle θ0=6.0 degrees, 8.5degrees, 11 degrees, 15.0 degrees, 17.5 degrees and 20 degrees. In moredetail, FIGS. 11A and 11B are graphs for evaluating color change of theoptical structures 100 corresponding to the aforementioned respectiveconcave-and-convex shapes 120, when the ratio β, which is a ratio of atotal length of the flat portions 121A, 122A with respect to a length ofone cycle of the concavity 121 and the convexity 122 of theconcave-and-convex shape 120, is 0.80.

FIG. 11A is a graph in which an axis of abscissa shows an angle in aviewing angle of light emerged from the optical structure 100, and anaxis of ordinate shows a color change Δu′v′. FIG. 11A shows color changein a viewing angle of light which was incident from the body (the sideof the liquid crystal panel 15) of the display device 10 and emergedfrom the optical structure 100. In the optical structures 100 under theabove respective conditions, the side surfaces 120S are set to have thesame curvature with each other. In addition, FIG. 11B is a graph inwhich an axis of abscissa shows a value of the average inclination angleθ0, and an axis ordinate shows a color change score. FIG. 11B shows acolor change score of light which was incident from the body (the sideof the liquid crystal panel 15) of the display device 10 and emergedfrom the optical structure 100. The color change score is calculated bythe aforementioned Expression (3).

In view of FIG. 11A created as described above, it can be understoodthat, when the display device 10 was provided with the optical structure100 in which the concave-and-convex shape 120 was formed such that theside surface 120S had the average inclination angle θ0 of 11 degrees, 15degrees, 17.5 degrees or 20 degrees, color change in a viewing angle wassuppressed, as compared with a case in which the display device 10 wasnot provided with the optical structure 100. On the other hand, when thedisplay device 10 was provided with the optical structure 100 in whichthe concave-and-convex shape 120 was formed such that the side surface120S had the average inclination angle θ0 of 6 degrees or 8.5 degrees,color change within a range of from 15 to 25 degrees was larger, ascompared with a case in which the optical structure 100 was notprovided. Thus, it can be understood that they are not preferred. Fromthis tendency, the present inventors have found that, when the averageinclination angle θ0 is too small, since an amount of light that istotally reflected by the side surface 120S reduces, the diffusion effectis weak so that the color change suppression effect may be insufficient.

On the other hand, in view of FIG. 11B, it can be understood that, whenthe average inclination angle θ0 is within a range of not less than 9degrees and not more than 18 degrees, the color change score is suitablysuppressed. From this point and the finding about the above-describeddifference in the color change, it can be concluded that, when theaverage inclination angle θ0 is within a range of not less than 9degrees and not more than 18 degrees, color change dispersion in aviewing angle is suitably suppressed.

From the above, the present inventors specify that a preferred range ofthe average inclination angle θ0 is not less than 9 degrees and not morethan 18 degrees. It was practically confirmed that, with such a range,the color change dispersion in a viewing angle could be effectivelysuppressed, as compared with a range outside this range. The averageinclination angle θ0 is preferably not less than 9 degrees and not morethan 18 degrees, and is more preferably not less than 10 degree and notmore than 17.5 degrees. Most preferably, the average inclination angleθ0 is not less than 11 degree and not more than 15 degrees. The colorchange graph shown in FIG. 11B varies depending on a value of the ratioβ which is a ratio of a total length of the flat portions 121A, 122Awith respect to a length of one cycle of the concavity 121 and theconvexity 122 of the concave-and-convex shape 120. However, the presentinventors have found that, when the average inclination angle θ0 is notless than 9 degrees and not more than 18 degrees, a good color changesuppression effect can be obtained irrespective of the value of theratio β.

In addition, FIG. 12A is a graph showing a radiance at a wavelength of450 nm in a viewing angle of light which was incident from the body (theside of the liquid crystal panel 15) of the display device 10 andemerged from the optical structure 100. An axis of abscissa shows anangle in a viewing angle, and an axis of ordinate shows a radiance. FIG.12B is a graph showing a degree of lowering of a radiance of light whichwas incident from the body (the side of the liquid crystal panel 15) ofthe display device 10 and emerged from the optical structures 100 underthe aforementioned conditions. An axis of abscissa shows a value of theaverage inclination angle θ0, and an axis of ordinate shows a radiancelowering rate (written as brightness lowering rate in FIG. 12B). Theradiance lowering rate is a barometer which shows that, when a valuethereof is smaller, a degree of lowering of a radiance is smaller, ascompared with a case in which the optical structure 100 is not provided.

In addition, FIG. 13A is a graph showing a contrast in a viewing angleof light which was incident from the body (the side of the liquidcrystal panel 15) of the display device 10 and emerged from the opticalstructure 100. An axis of abscissa shows an angle in a viewing angle,and an axis of ordinate shows a contrast. FIG. 13B is a graph showing adegree of lowering of contrast of light which was incident from the body(the side of the liquid crystal panel 15) of the display device 10 andemerged from the optical structures 100 under the aforementionedconditions. An axis of abscissa shows a value of the average inclinationangle θ0, and an axis of ordinate shows a contrast lowering rate. Thecontrast lowering rate is a barometer which shows that, when a valuethereof smaller, a degree of lowering of contrast is smaller, ascompared with a case in which the optical structure 100 is not provided.

From the result of FIGS. 12A to 13B, it was found that, when the averageinclination angle θ0 is not less than 9 degrees and not more than 18degrees, there is a trade-off relationship between thebrightness/contrast and color change.

(Relationship Between Ratio of Flat Portions of Concave-and-Convex Shapeand Color Change)

Next, the reason why the ratio 13, which is a ratio of a total value ofwidths of the flat portions 121A, 122A with respect to a length of onecycle of the concavity 121 and the convexity 122 of theconcave-and-convex shape 120, is particularly preferably not less than0.74 and not more than 0.83, when the inclination angle range α is 12degrees by way of example, is described. As described above, based onthe intensive studies, the present inventors have found that, in theoptical structure 100, the ratio β within a range of not less than 0.60and not more than 0.90. Herebelow, the fact that, when the inclinationangle α is 12 degrees, the ratio β within a range of not less than 0.74and not more than 0.83 is particularly preferred is described based onexperiments or simulations. Graphs shown in FIGS. 14A and 14B are graphswhich show, when the inclination angle range α is 12 degrees, colorchange in a viewing angle of optical structures 100 corresponding to theconcave-and-convex shapes 120 in which the ratio β is 0.90, 0.80, 0.75and 0.63. FIG. 14A is a graph in which an axis of abscissa shows anangle in a viewing angle of light emerged from the optical structure100, and an axis of ordinate shows a color change Δu′v′. FIG. 14A showscolor change in a viewing angle of light which was incident from thebody (the side of the liquid crystal panel 15) of the display device 10and emerged from the optical structure 100. In addition, FIG. 14B is agraph in which an axis of abscissa shows a value of the ratio β, and anaxis ordinate shows a color change score. FIG. 14B shows a color changescore of light which was incident from the body (the side of the liquidcrystal panel 15) of the display device 10 and emerged from the opticalstructure 100. The color change score is calculated by theaforementioned Expression (3).

In view of FIG. 14A, it can be understood that, when the display device10 was provided with the optical structure 100 in which theconcave-and-convex shape 120 was formed to have the side surface 120S inwhich the ratio β of the flat portions was 0.63, 0.75, 0.80 and 0.90,color change in a viewing angle was suppressed, as compared with a casein which the display device 10 was not provided with the opticalstructure 100. However, it can be understood that, in the case of theconcave-and-convex shape 120 having the side surface 120S in which theratio β is 0.90, color change degree is smaller, with respect to a casein which the display device 10 was not provided with the opticalstructure 100. From this tendency, the present inventors have foundthat, when the ratio β is too large, the diffusion effect by the sidesurface 120S is weak so that the color change suppression effect may beinsufficient.

On the other hand, in view of FIG. 14B, it can be understood that, whenthe ratio β of the flat portions is 0.63, 0.75, 0.80 and 0.90, colorchange in a viewing angle is suppressed. However, when the ratio β is0.90, the color change score is relatively high, i.e., the color changeis evaluated to be relatively large. In addition, when the ratio β is0.63, the color change score is low, i.e., the color change is evaluatedto be small. However, regarding the color change, there is no continuityto a value larger than this value “0.63”, whereby it cannot be said thata stable color change suppression effect can be obtained. FIG. 14B alsoshows a color change score in the optical structure 100 in which theratio β of the flat portions is different from ones illustrated in FIG.14A. From the above points, the present inventors have concluded that,when the inclination angle range α is 12 degrees and the ratio β iswithin a range of not less than 0.70 and not more than 0.86, colorchange dispersion in a viewing angle can be stably suppressed. Inconsideration of the graphs, it is considered that the ratio β of notless than 0.74 and not more than 0.83 is particularly preferred.

In addition, FIG. 15A is a graph showing a radiance at a wavelength of450 nm in a viewing angle of light which was incident from the body (theside of the liquid crystal panel 15) of the display device 10 andemerged from the optical structure 100. An axis of abscissa shows anangle in a viewing angle, and an axis of ordinate shows a radiance. FIG.15B is a graph showing a degree of lowering of a radiance of light whichwas incident from the body (the side of the liquid crystal panel 15) ofthe display device 10 and emerged from the optical structures 100 underthe aforementioned conditions. An axis of abscissa shows a value of theratio β, and an axis of ordinate shows a radiance lowering rate (writtenas brightness lowering rate in FIG. 15B). The radiance lowering rate isa barometer which shows that, when a value thereof is smaller, a degreeof lowering of a radiance is smaller, as compared with a case in whichthe optical structure 100 is not provided.

In addition, FIG. 16A is a graph showing a contrast in a viewing angleof light which was incident from the body (the side of the liquidcrystal panel 15) of the display device 10 and emerged from the opticalstructure 100. An axis of abscissa shows an angle in a viewing angle,and an axis of ordinate shows a contrast. FIG. 16B is a graph showing adegree of lowering of contrast of light which was incident from the body(the side of the liquid crystal panel 15) of the display device 10 andemerged from the optical structures 100 under the aforementionedconditions. An axis of abscissa shows a value of the ratio β, and anaxis of ordinate shows a contrast lowering rate. The contrast loweringrate is a barometer which shows that, when a value thereof smaller, adegree of lowering of contrast is smaller, as compared with a case inwhich the optical structure 100 is not provided.

When the aforementioned ratio β is within a range of not less than 0.74and not more than 0.83, it can be concluded that lowering of abrightness and a contrast is not excessive, as compared with a case inwhich the optical structure 100 is not provided. Thus, within thisrange, color change in a viewing angle can be effectively suppressed,while maintaining good display quality of the display device 10 in afront view. Also from this point, it can be said that the ratio β ispreferably not less than 0.74 and not more than 0.83.

In addition, as described above, when the inclination angle range α is12 degrees, a particularly preferred range of the ratio β is not morethan 0.74 and not less than 0.83. On the other hand, as described in theaforementioned “Relationship between Inclination Angle Range (Curvature)of Side Surface of Concave-and-convex shape and Color Change”, when theratio β is 0.80, a particularly preferred range of the inclination anglerange α is not less than 9 degrees and not more than 16 degrees. Fromthe intensive studies on these results, the present inventors havereached a relationship in which a preferred value of the inclinationangle range α varies depending on a value of the ratio 13.

The inclination angle range α is preferably not less than 83.33β-59.67degrees and not more than 80β-44 degrees, and more preferably not lessthan 66.6β-37.33 degrees and not more than 100β-71 degrees. Note that ais more than 0 and that μ is less than 1. From the above expressions, itcan be said that μ is preferably not less than 0.55 and less than 1.00,and that α is preferably greater than 0 and less than 36 degrees.

When the optical structure 100 is manufactured in accordance with theseconditions, it is possible to obtain the optical structure 100 capableof simply suppressing color change dispersion in a viewing angle whilemaintaining good display quality of a display device in a front view.

In addition, in the structure of the display device 10 according to thisembodiment, light from the surface light source device 20 is incident onthe back surface of the optical structure 100. At this time, even when arange of an emission angle of the light from the surface light sourcedevice 20 is not less than 0 degrees and not more than 90 degrees, anangular range of a light ray inside the optical structure 100 isnarrower than an emission angle range of the light from the surfacelight source device 100. This is because, when the light is incident onthe optical structure 100, the light is refracted. In the structure ofthe display device 10 according to this embodiment, when an emissionangle range of light from the surface light source device 20 is not lessthan 0 degree and not more than 90 degrees, it is assumed that anangular range of a light ray inside the optical structure 100 is nearlynot less than 0 degrees and not more than 60 degrees, although itdepends on refractive indexes of the surface light source device 20 andthe optical structure 100. To be specific, when a refractive index ofthe high refractive-index layer 102 is not less than 1.15, an angularrange of a light ray inside the optical structure 100 may be nearly notless than 0 degrees and not more than 60 degrees. In addition, when arefractive index of the high refractive-index layer 102 is 1.29, anangular range of a light ray inside the optical structure 100 can belowered down to not less than 0 degrees and not more than 51 degrees.Further, when a refractive index of the high refractive-index layer 102is not less than 1.6, which is more easily manufactured, an angularrange of a light ray inside the optical structure 100 can be lowereddown to not less than 0 degrees and not more than 40 degrees.

It is not necessary that the side surface 120S of the concave-and-convexshape 120 exerts an optical effect on light of an angle larger than amaximum angle of an angular range of a light ray inside the opticalstructure 100. Based on this point, in consideration of a maximum angleof an angular range of a light ray inside the optical structure 100, theinclination angle range α may be greater than 0 degrees and not morethan 60 degrees. Depending on a refractive index value of the highrefractive-index layer 102, the inclination angle range α may be greaterthan 0 degrees and not more than 40 degrees. The present inventors haveconfirmed that, when the inclination angle range α is not less than 3degrees, a color change suppression effect can be visually recognized.From this point, the inclination angle range α is preferably not lessthan 3 degrees and not more than 60 degrees. Depending on a refractiveindex value of the high refractive-index layer 102, the inclinationangle range α is more preferably not less than 3 degrees and not morethan 40 degrees. Note that the high refractive-index layer in thepresent invention is referred to as high refractive-index layer becauseit has a refractive index higher than that of the low refractive-indexlayer.

(Behavior of Light in Display Device)

Next, a behavior of light in the display device 10 according to thisembodiment is described with reference again to FIG. 2. In order todisplay an image on the display device 10 according to this embodiment,light is firstly emitted from the light source 24. As shown in FIG. 2,the light incident from the incident surface 33 into the light guideplate 30 is guided in the light guide plate 30 substantially along thefirst direction d₁ toward the opposite surface 34 which is opposed tothe incident surface 33 along the first direction d₁. The guided lightrepeats total reflection between the main surfaces 31, 32 of the lightguide plate 30. When an incident angle of the light to the main surface31 becomes less than a total reflection critical angle, the lightemerges from the light guide plate 30, as shown by L1 of FIG. 2. Whenthe light emerged from the light guide plate 30 passes through theoptical sheet 60, a traveling direction and a polarization state of thelight are converted by the unit prisms 70 into desired ones. Then, thelight is incident on the liquid crystal panel 15. Then, in the liquidcrystal layer 12, transmission or block of the light incident on theliquid crystal panel 15 are controlled for each area that forms a pixel,in accordance with application of voltage. Thus, an image is displayedon the display surface 15A of the liquid crystal panel 15.

In this embodiment, the light emerged from the display surface 15A ofthe liquid crystal panel 15 is incident on the optical structure 100. Atthis time, the light incident from the side of the liquid crystal panel15 into the optical structure 100 is subjected to an optical effectcaused by transmission and total reflection by means of theconcave-and-convex shape 120. Namely, at this time, the light thattravels on a high angle side of a viewing angle is totally reflected soas to be diffused widely in directions including a low angle side, bythe side surface 120S of the concave-and-convex shape 120, which is acurved surface that is convex to the low refractive-index layer 103. Inaddition, the light that is perpendicularly incident on the opticalstructure 100 travels in the front direction by the flat portions 121A,122A of the concave-and-convex shape 120 so that its diffusion issuppressed. Thus, color change in a viewing angle is suppressed, andlowering of brightness and contrast in a front view is also suppressed.

Therefore, this embodiment can simply suppressing color changedispersion in a viewing angle, while maintaining good display quality ofthe display device in a front view. In addition, in this embodiment,when the aforementioned ratio β is 0.80, a difference (inclination anglerange α) between a maximum angle and a minimum angle, which are definedbetween the side surface 120S of the concave-and-convex shape 120 andthe normal direction of the high refractive-index layer 102 and the lowrefractive-index layer 103, is not less than 9 degrees and not more than16 degrees, for example, excessive lowering of brightness and contractban be suppressed. In addition, when the average inclination angle θ0 ofthe side surface 120S, which is defined between a straight lineconnecting both end points of the side surface 120S of theconcave-and-convex shape 120 and the normal direction of the highrefractive-index layer 102 and low refractive-index layer 103, is notless than 9 degrees and not more than 18 degrees, color changedispersion can be effectively suppressed. Note that such a numericalrange is nothing more than an example of a particularly preferred range,and that the present invention can achieve a useful effect even with anumerical range different from the illustrated one.

In addition, as described above, the flat portions 121A, 122A of theconcave-and-convex shape 120 have a function for transmitting light,which is perpendicularly incident on the optical structure 100 in afront direction. Thus, in this embodiment, diffusion of emergent lightis suppressed, so that lowering of brightness and contrast in a frontview is suppressed. On the other hand, the concave-and-convex shape 120having the flat portions 121A, 122A is advantageous in terms ofmanufacture. To be specific, even when a ratio between a length of theflat portion 121A and a length of the flat portion 122A is optionallychanged to some degree under a condition where the shape of the sidesurface 120S of the concave-and-convex shape is fixed, an opticalproperty is almost unchanged. Thus, in a design stage, the ratio β ofthe flat portions 121A, 122B with respect to one cycle of the concavity121 and the convexity 122 of the concave-and-convex shape 120 is fixedat a desired value, and a ratio between a length of the flat portion121A and a length of the flat portion 122A is optionally changed to somedegree, so that it is possible to manufacture the optical structure 100having a desired optical property by a desired manufacturing process. Tobe specific, when a ratio of the flat portion 121A of the concavity 121is large, it is easy to fill the low refractive-index layer 103. When aratio of the flat portion 122A of the convexity 122 is large, diecutting of the high refractive-index layer 102 is easy. In addition,when the high refractive-index layer 102 is shaped, a problem of mixtureof air can be suppressed. It was found that, when a ratio between theflat portion 121A and the flat portion 122A is 7:4, for example, such amanufacturing advantage can be obtained. When a ratio between the flatportion 121A and the flat portion 122A is changed, an optical propertyis almost unchanged excluding an extreme case. For example, in a casewhere the inclination angle range α=12, the ratio β=0.80 and the averageinclination angle θ0=11, when a ratio between the flat portions 121A and122A was changed within 7:3 to 3:7, an optical property was almostunchanged. On the other hand, a ratio of the width a of the flat portion122A of the convexity 122 with respect to the pitch P is preferably notless than 0.19 and not more than 0.45. As apparent from FIG. 6, when thewidth a of the flat portion 122A is too small, light incident on theside surface 120S tends to decrease. The present inventors haveconfirmed through experiments that, when a ratio of the width a of theflat portion 122A of the convexity 122 with respect to the pitch P isnot less than 0.19 and not more than 0.45, color change can beeffectively suppressed.

In consideration of easy die cutting, the larger the ratio β is, themore easily die cutting becomes. In consideration of such amanufacturing advantage, the ratio β is preferably not less than 0.50and less than 1.00. In consideration that α color change suppressioneffect can be easy ensured, the ratio β is preferably not less than 0.60and less than 0.90.

In addition, in this embodiment, since a refractive index of theantireflection layer 104 serving as a surface member is not more than1.40, a critical angle, at which light coming from the display device 10starts to totally reflect at an interface between the antireflectionlayer 104 and air, is larger than a case where a general surfacematerial having a refraction index of about 1.40 to 150 is used. Thus,since an amount of light that can be take out can be increased ascompared with a general structure, it can be suppressed that αbrightness in a viewing angle is undesirably lowered. As shown in FIG.8A, for example, when the optical structure 100 is used, a brightness ina front view is somewhat lowered. However, in this embodiment, since theantireflection layer 104 having a refractive index of not more than 1.40increases an amount of light that can be taken out, so that lowering ofbrightness in a front view can be compensated. Thus, good displayquality in the whole viewing angle can be achieved.

Although the embodiment of the present invention and its modificationexample have been described above, the present invention is not limitedto the above embodiment, and the embodiment and its modification examplecan be further modified. For example, in the above embodiment, thesurface light source device 20 is of an edge light type, for example.However, the surface light source device 20 may be of a direct type.

In addition, in the above embodiment, the low refractive-index layer 103functions as an adhesive layer, and is joined to the display surface 15Aof the liquid crystal panel 15. However, as shown in FIG. 17, the lowrefractive-index layer 103 may be joined to the display surface 15A ofthe liquid crystal panel 15 through an adhesive layer 105. In FIG. 17, asectional structure of the upper polarizing plate 13 is shown in detail.The upper polarizing plate 13 shown in FIG. 17 is formed by disposing apolarizing plate body 13B between a pair of support layers 13A made oftriacetyl cellulose or the like. Alternatively, the low refractive-indexlayer 103 may be joined to the display surface 15A of the liquid crystalpanel 15 r through an adhesive layer.

In addition, in the above embodiment, the base member 101 is disposedbetween the high refractive-index layer 102 and the antireflection layer104. However, as shown in FIG. 18, the antireflection layer 104 may beprovided directly on the high refractive-index layer 102.

In addition, in the above embodiment, the high refractive-index layer102 and the low refractive-index layer 103 are disposed nearer to thelight emergent side than the upper polarizing plate 13. However, asshown in FIG. 19, the high refractive-index layer 102 and the lowrefractive-index layer 103 may be disposed between the upper polarizingplate 13 and the liquid crystal layer 12.

In addition, in the above embodiment, only the structure in which theside surface S is a curved surface defining an arc of a precise circleis shown. However, as shown in FIG. 20, the side surface 120S may beformed along an arc of an ellipse. In this case, the “inclination anglerange α”, which is defined by a difference between the maximum angle θ2and the minimum angle θ1, which are defined between the side surface Sand a normal direction of the high refractive-index layer 102 and thelow refractive-index layer 103, can be increased, an advantage in thatthe side surface 120S capable of diffusion light widely can be madecompact can be obtained.

-   10 Display device-   12 Liquid crystal layer-   13 Upper polarizing plate-   14 Lower polarizing plate-   15 Liquid crystal panel-   15A Display surface-   15B Back surface-   20 Surface light source device-   21 Light-emitting surface-   24 Light source-   25 Point-like light-emitting element-   28 Reflection sheet-   30 Light guide plate-   60 Optical sheet-   100 Optical structure-   101 Base member-   101A Emergent surface-   101B Back surface-   102 high refractive-index layer-   103 low refractive-index layer-   104 Antireflection layer-   110 Lens part-   120 Concave-and-convex shape-   120S Side surface-   121 Concavity-   121A Flat portion-   122 Convexity-   122A Flat portion

What is claimed is:
 1. An optical structure to be disposed on a displaysurface of a display device, comprising: a high refractive-index layer;and a low refractive-index layer laminated on the high refractive-indexlayer, and having a refractive index lower than that of the highrefractive-index layer; wherein: an interface between the highrefractive-index layer and the low refractive-index layer has aconcave-and-convex shape; each of a concavity and a convexity in theconcave-and-convex shape has a flat portion extending in a surfacedirection of the high refractive-index layer and the lowrefractive-index layer; a side surface of the concave-and-convex shape,which extends between the flat portion of the concavity and the flatportion of the convexity, is a curved surface or a folded surface thatis convex towards the low refractive-index layer side from the highrefractive-index layer side; the low refractive-index layer isconfigured to be disposed to face the display surface of the displaydevice; and a difference between a maximum angle and a minimum angle,which are defined between the side surface of the concave-and-convexshape and a normal direction of the high refractive-index layer and thelow refractive-index layer, is not less than 3 degrees and not more than60 degrees.
 2. An optical structure to be disposed on a display surfaceof a display device, comprising: a high refractive-index layer; and alow refractive-index layer laminated on the high refractive-index layer,and having a refractive index lower than that of the highrefractive-index layer; wherein: an interface between the highrefractive-index layer and the low refractive-index layer has aconcave-and-convex shape; each of a concavity and a convexity in theconcave-and-convex shape has a flat portion extending in a surfacedirection of the high refractive-index layer and the lowrefractive-index layer; a side surface of the concave-and-convex shape,which extends between the flat portion of the concavity and the flatportion of the convexity, is a curved surface or a folded surface thatis convex to towards the low refractive-index layer side from the highrefractive-index layer side; the low refractive-index layer isconfigured to be disposed to face the display surface of the displaydevice; and a ratio of a total length of the flat portions with respectto a length of one cycle of the concavity and the convexity of theconcave-and-convex shape is not less than 0.50 and less than 1.00.
 3. Anoptical structure to be disposed on a display surface of a displaydevice, comprising: a high refractive-index layer; and a lowrefractive-index layer laminated on the high refractive-index layer, andhaving a refractive index lower than that of the high refractive-indexlayer; wherein: an interface between the high refractive-index layer andthe low refractive-index layer has a concave-and-convex shape; each of aconcavity and a convexity in the concave-and-convex shape has a flatportion extending in a surface direction of the high refractive-indexlayer and the low refractive-index layer; a side surface of theconcave-and-convex shape, which extends between the flat portion of theconcavity and the flat portion of the convexity, is a curved surface ora folded surface that is convex to towards the low refractive-indexlayer side from the high refractive-index layer side; the lowrefractive-index layer is configured to be disposed to face the displaysurface of the display device; and an average inclination angle of theside surface of the concave-and-convex shape, which is defined between astraight line connecting both end points of the side surface of theconcave-and-convex shape and a normal direction of the highrefractive-index layer and the low refractive-index layer, is not lessthan 9 degrees and not more than 18 degrees.
 4. A display device inwhich the optical structure according to claim 1 is disposed on adisplay surface.
 5. The display device according to claim 4, comprising:a liquid crystal panel having the display surface and a back surfaceopposed to the display surface; and a surface light source devicedisposed to face a back surface of the liquid crystal panel.
 6. Thedisplay device according to claim 5, wherein the liquid crystal panel isa VA type liquid crystal panel which is configured such that, when avoltage to liquid crystal molecules is off or at a minimum value, theliquid crystal molecules are oriented along a normal direction of thedisplay surface so that light from the surface light source device isblocked, and such that, when a voltage to the liquid crystal moleculesis gradually increased, the liquid crystal molecules are inclined littleby little to a side along the display surface so that a transmittance ofthe light from the surface light source device is gradually increased.